Consumers first judge the quality of the food product by its color, at least according to Food Technology, page 49 (July, 1986). The food industry has catered to consumer aesthetics, if not actually fostering such an attitude, by giving careful attention to the color of their products, including conducting ongoing investigations into materials which may be used as suitable food colorants.
Although naturally occurring pigments perforce were the first used food colorants, the development of chemistry as a discipline led to many synthetic dyes, especially anilines, to supplant naturally occurring pigments as food additives. As a class synthetic colorants have many advantages, such as a uniform and reproducible color, color stability, absence of flavor, and an oxidative and/or thermal and/or photostability superior to naturally occurring pigments, broad availability relatively insensitive to changes in crop yields and so forth. The resulting popularity of synthetic colorants at least is understandable.
However, with heightened awareness of a consuming public to food additives and increased testing of some representative examples came a concern about their safety. Recent years have seen some materials formerly used as food colorants run the gamut from being beyond reproach to being suspect and even banned or at least used restrictedly. For example, FD&C Red No. 2 and FD&C Violet No. 1 have been banned in the United States and many other countries. Because of a variety of allergic reactions in sensitive individuals induced by FD&C Yellow No. 5 a recent ruling by the FDA requires food colored with it be declared as such on product labels. As a consequence the pendulum has begun to swing once more toward naturally occurring pigments as food additives.
The major pigments produced by Monascus species traditionally grown on rice in the Orient are orange and relatively insoluble in water, but readily react with compounds containing amino groups to form water soluble colorants. Monascus pigments have been used in the Orient for hundreds of years as a general food colorant and as a colorant for wine and bean curd. They can be made water soluble or oil soluble and are stable at a pH range 2-10. They are heat stable and can be autoclaved. In oriental countries microorganisms of this type typically are grown on grains of rice and once the grains have been penetrated by the red mycelium the whole mass is finely ground with the resulting powder used as a food colorant.
Monascus species have been reported to elaborate several pigments, but most species seem to produce an orange pigment as the major colorant. This water-insoluble pigment is a mixture of monascorubrin and rubropunctatin, whose structures were elucidated by B. C. Fielding et al., Tetrahedron Letters, No. 5, 24-7 (1960) and Kumasaki et al., Tetrahedron, 18, 1171 (1962), and which differ in the former having a 7-carbon chain attached to the ketonic carbonyl group and the latter having a 5-carbon chain. At least some species, notably M. purpureus, produce a yellow pigment, monascoflavin, the reaction product of rubropunctatin with two moles of hydrogen and which arises from reduction of two conjugated olefinic bonds in the chromophore of the parent. Y. Inouye et al., Tetrahedron, 18, 1195 (1962). [Parenthetically, it may be noted that these authors state that monascoflavin is the reduction product of monascorubrin. However, monascorubrin and rubropunctatin are homologs differing in having C.sub.7 H.sub.15 and C.sub.5 H.sub.11 ketonic side chains, respectively, and monascoflavin is specified as having a C.sub.5 H.sub.11 side chain. Therefore its precursor must be rubropunctatin. It must be realized that for many years there was rampant confusion between monascorubrin and rubropunctatin, with a concomitant lack of distinction, whose effects are not yet entirely dispelled.]
Although the monascorubrin-rubropunctatin mixture which constitutes the orange pigment produced as the direct fermentation product of Monascus species is water insoluble and therefore is of limited utility as a food colorant, it has been recognized for some time that these materials react with primary amines to afford red colorants, many of which are water soluble. Yamaguchi, U.S. Pat. No. 3,765,906, reported that the orange insoluble pigment, either in the fermentation medium or as an isolate, reacted with water-soluble proteins, peptides, or amino acids to afford red water-soluble pigment. The reaction of the orange water-insoluble pigment with amino sugars, polymers of amino sugars, polyamino acids, and amino alcohols is reported in U.S. Pat. No. 3,993,789. The production of red water-soluble pigment by reacting the insoluble orange pigment with aminoacetic acid and aminobenzoic acid has been reported by Wong and Koehler, J. Food Science, 48, 1200 (1983), who also investigated their color characteristics and stability. All of the aforementioned water-soluble red pigments are believed to have the structure II, ##STR1## where I is a monascorubrin-rubropunctatin mixture and R.sub.1 is C.sub.5 H.sub.11 or C.sub.7 H.sub.15. Despite the interest as manifested by the numerous citations, none of the red water-soluble pigments appear to have gained broad, substantial use as a food colorant.
As previously mentioned, some Monascus species form as a fermentation product water-insoluble yellow pigment whose structure appears not to have been elucidated but which is believed to be the reduction product of rubropunctatin and monascorubrin where two of the carbon-carbon double bonds in the conjugated chain are reduced to carbon-carbon single bonds. In the course of their structure elucidation Kumasaki et al., op. cit., produced several different reduction products of monascorubrin-rubropunctatin and their ammonia adducts, depending upon the reducing method employed. In contrast to other reducing agents, sodium borohydride reduction of the ammonia adduct of the orange water-insoluble pigment effected specific reduction of the carbonyl group of the isoquinoline nucleus to an hydroxyl moiety, an observation to which we shall shortly return.
In the course of a related investigation we had occasion to reduce some amine reaction products, II, of monascorubrin-rubropunctatin mixtures using sodium borohydride and noticed this was accompanied by a color change from red to yellow. Since the replacement of the coal tar dye known as FD+C Yellow No. 5 currently is highly desirable, this observation spurred us to further investigate our reduction products; merely because a compound is yellow does not automatically qualify it as a food colorant candidate. A putative yellow food colorant needs to have adequate water solubility, needs to be color stable over a range of pH, should have a yellow color of a hue acceptable to the subjective standards current to the food industry and of an intensity relatively high to permit its use at low concentrations, and have no significant objectionable taste, preferably no taste at all, at a concentration effective to color food. The food colorants of our invention meet all these criteria.